1. Field of the Invention
The present invention relates generally to a knee joint prosthesis, which replaces the articulating knee portion of the femur and tibia, and more particularly, to a knee joint prosthesis system, which includes elongated femoral components for posterior stabilized and fully constrained applications.
2. Discussion of the Related Art
The knee joint is a complex articulating structure. The knee joint includes a femur, which articulates with a tibia, and a patella, which acts as a protective shield for the articulating knee joint. The knee joint also includes soft tissue ligaments, which extend on the medial and lateral side of the knee joint, which are generally referred to as collateral ligaments and ligaments which cross within the knee joint generally referred to as an anterior cruciate ligament and a posterior cruciate ligament.
A knee joint prosthesis typically comprises a femoral component and a tibial component. The femoral component and the tibial component are designed to be surgically attached to the distal end of the femur and the proximal end of the tibia, respectively. The femoral component is further designed to cooperate with the tibial component in simulating and articulating motion of an anatomical knee joint.
The motion of a natural knee joint is kinematically complex. During a relatively broad range of flexion and extension, the articular or bearing surfaces of a natural knee experience rotation, medial and lateral angulation, translation in the sagittal plane, rollback and sliding. Knee joint prostheses, in combination with ligaments and muscles, attempt to duplicate this natural knee motion, as well as absorb and control forces generated during the full range of flexion. Depending on the degree of damage or deterioration of the knee tendons and ligaments, it may be necessary for a knee joint prosthesis to eliminate one or more of these motions in order to provide adequate stability.
To provide for these stabilities, knee joint prostheses generally have different levels of constraint. For example, cruciate retaining (CR) knee joint prostheses provide the least level of constraint, posterior stabilized (PS) knee joint prostheses provide an intermediate level of constraint, while fully constrained knee joint prostheses provide the highest level of constraint upon the kinematic motions of a knee joint. In some situations, a patient may initially require a less constrained knee joint, such as a cruciate retaining (CR) or posterior stabilized (PS) knee joint prosthesis. The patient may later require a fully constrained knee joint prosthesis because the patient is exhibiting instability. Moreover, during the surgical procedure, a surgeon may initially wish to implant a cruciate retaining type knee joint prosthesis and subsequently realize that a posterior stabilized or a fully constrained knee joint prosthesis is required, which may lead to additional surgical resections, as well as surgical time and cost.
There is much debate among knee surgeons regarding whether to resect the posterior cruciate ligament (PCL). Some surgeons will preserve the PCL if at all possible. Many other surgeons will selectively preserve the PCL. Some surgeons who sacrifice the PCL may rely on flexion-extension balance and tibial inserts for stability. Other surgeons may advocate PCL excision and substitute for the ligament using a posterior stabilized (PS) knee joint prosthesis.
PCL substitution with a posterior stabilized (PS) knee joint prosthesis generally requires the removal of intercondylar bone for the PS knee joint prosthesis. As a consequence, there is increased bone removal compared with PCL preservation and the use of a cruciate retaining CR knee joint prosthesis. It has also been suggested in the literature that removal of intercondylar bone may predispose to intercondylar bone fracture. Additionally, resection of the PCL results in an increase in flexion gap relative to extension gap. In contrast to this, the surgical technique for PCL retention is sometimes less forgiving than for PCL substitution because, in addition to balancing the flexion and extension gaps and collateral ligaments, balancing of the PCL is required. In some patients, the PCL may be absent or incompetent and balancing is, therefore, difficult to achieve.
Despite the PCL debate, there appears to be no clear advantage to PCL preserving or substituting designs with regard to knee joint prosthesis. Clinical results appear to indicate that success of knee joint prostheses is associated with factors other than whether or not the PCL is preserved.
Normally, a surgeon will preoperatively select a certain type of knee joint prosthesis. However, the surgeon may find interoperatively in trial, that the preselected prosthesis may not provide the best function of the knee joint and that surgical adjustments in the knee joint are required. In this case, the surgeon is faced with several disadvantages, which will be illustrated in the examples below.
In one example, the surgeon may decide preoperatively to use a posterior stabilized PS knee joint prosthesis. Then, interoperatively in trial if the surgeon determines that the increase in flexion gap is minimal, the surgeon may decide not to compensate for the increased flexion gap. However, if the increase in flexion gap is greater than the surgeon had interoperatively evaluated it to be, the knee joint may postoperatively become loose in flexion which can affect the stability and balance of the implanted knee joint. Additionally, the remaining ligaments in the patients knee joint may become lax and this can also affect stability and wear in the prosthetic knee joint. This is because there may be more edge loading or rotation in the bearing of the prosthetic knee joint and that may be disadvantageous for the knee joint prosthesis patient in the long term.
In another situation, the surgeon may decide interoperatively to change from a cruciate retaining (CR) knee joint prosthesis to a posterior stabilized (PS) knee joint prosthesis. However, by removing the PCL, the flexion gap will increase and further bone resection may be required. To address this situation, the surgeon may opt to use a larger femoral component. However, the larger femoral component places the articulation point of the knee joint in the same position relative to the anterior cortex and by using the larger femoral component, it will occupy the region of the extra flexion gap. But the width in the medial-lateral (ML) portion increases with the larger femoral component. This can restrict flexion of the knee, which might not be acceptable to an active knee joint prosthesis patient.
In still another example, where the surgeon decides to interoperatively change from a cruciate retaining (CR) knee joint prosthesis to a posterior stabilized (PS) knee joint prosthesis and the femur has already been resected to form the engagement surface, the surgeon will generally be unable to use a larger femoral member without using a posterior augment. Currently, many surgeons are reluctant to use a posterior augment in the knee for fear of future deterioration of the knee joint. Additionally, if there is an increase in flexion gap, the surgeon may decide to insert a thicker bearing to decrease the flexion gap. The end result is that in extending the knee position, the patient may complain of a xe2x80x9ctight kneexe2x80x9d. Anticipating this, a surgeon may resect the femoral distal bone and move the femoral component up the leg to obtain an equal gap in flexion and extension. However, this may create problems with patella tracking in the knee joint and the knee joint prosthesis patient may find this to be disadvantageous.
What is needed then is a knee joint prosthesis system which does not suffer from the above mentioned disadvantages. This, in turn, will permit interoperative options for selecting a cruciate retaining (CR) knee joint prosthesis, a posterior stabilized (PS) knee joint prosthesis or a fully constrained knee joint prosthesis where the posterior stabilized and the fully constrained knee joint prosthesis compensate for the increased flexion gap due to resection of the posterior cruciate ligament, reduce or eliminate the requirement for further bone resection irrespective of the type of constraint knee joint prosthesis selected, thereby reducing surgical time, cost and complexity, and further provide selection between the various constrained knee joint prostheses without having to oversize or undersize any of the particular components of the knee joint prosthesis. It is, therefore, an object of the present invention to provide an elongated femoral component in a knee joint prosthesis system that achieves the above-identified advantages.
In a first preferred embodiment, a knee joint prosthesis system adapted to replace the articulating knee portion of a femur and a tibia includes a first femoral component and a second femoral component. The first femoral component includes a first femoral engagement region and a first condylar portion having a first femoral bearing surface. The first femoral engagement region is operable to engage a resected engagement surface of the femur and the first condylar portion includes a first posterior region having a first thickness. The second femoral component includes a second femoral engagement region and a second condylar portion having a second femoral bearing surface. The second femoral engagement region is operable to engage the resected engagement surface of the femur and the second condylar portion includes a second posterior region having a second thickness. The second thickness is larger than the first thickness and a first femoral engagement region is substantially the same as the second femoral engagement region, such that a surgeon may select one of the first femoral component and the second femoral component for attachment to the femur.
In another preferred embodiment, a knee joint prosthesis adapted to replace the articulating knee portion of a femur and a tibia includes a cruciate retaining (CR) femoral component and a posterior stabilized (PS) femoral component. The cruciate retaining (CR) femoral component includes a first femoral engagement region and a first condylar portion having a first femoral bearing surface. The first femoral engagement region is operable to engage a resected engagement surface of the femur and the first condylar portion includes a first posterior region having a first thickness and a first distal region having a second thickness where the first thickness is substantially equal to the second thickness. The posterior stabilized (PS) femoral component includes a second femoral engagement region and a second condylar portion having a second femoral bearing surface. The second femoral engagement region is operable to engage the resected engagement surface of the femur and the second condylar portion includes a second posterior region having a third thickness and a second distal region having a fourth thickness. The third thickness is greater than the fourth thickness and the first femoral engagement region is substantially the same as the second femoral engagement region. This enables a surgeon to select the cruciate retaining (CR) femoral component or the posterior stabilized (PS) femoral component, whereby the posterior stabilized (PS) femoral component will accommodate for an increased flexion gap upon resection of a posterior cruciate ligament.
In yet another preferred embodiment, a method for implanting a femoral component to a femur having a resected engagement surface from a knee joint prosthesis system is provided. This method includes resecting the femur to provide the resected engagement surface and providing a first femoral component and a second femoral component. The first femoral component includes a first condylar portion which includes a first posterior region having a first thickness and a second femoral component includes a second condylar portion having a second posterior region having a second thickness where the second thickness is greater than the first thickness. The method further includes interoperatively determining if a posterior cruciate ligament should be removed and if the posterior cruciate ligament is retained selecting the first femoral component and if the posterior cruciate ligament is removed selecting the second femoral component which will accommodate for an increased flexion gap upon resection of the posterior cruciate ligament.
The use of the present invention provides a knee joint prosthesis system which includes an elongated femoral component to accommodate for an increased flexion gap upon resection of a posterior cruciate ligament. As a result, the aforementioned disadvantages associated with the currently available knee joint prosthesis have been substantially reduced or eliminated.